Every blink deposits a fresh tear film across the cornea. What looks like a single transparent layer is in fact a precisely engineered three-layer structure — each layer with a distinct molecular composition, a distinct cellular origin, and a distinct physiological function. When any layer fails, the whole film becomes unstable. That instability is dry eye disease.

The tear film refreshes approximately every 3–12 seconds with each blink and evaporates continuously between blinks. The ratio between production rate and evaporation rate determines tear film stability — and this balance is what the disease disrupts.

The critical vulnerability is the lipid layer. Produced exclusively by meibomian glands — vertical sebaceous glands embedded in the tarsal plates of both upper and lower eyelids — the lipid layer is a complex mixture of wax esters, cholesterol esters, and polar lipids. It functions as a surfactant and waterproofing barrier.

When meibomian glands become obstructed, atrophied, or produce abnormally thick secretions (as in MGD), the lipid layer thins. Tear evaporation accelerates. Osmolarity rises. The hyperosmolar tear film triggers an inflammatory cascade on the corneal and conjunctival epithelium — releasing cytokines (IL-1β, IL-6, IL-8, TNF-α) and matrix metalloproteinases that damage goblet cells, reducing mucin production, further destabilising the tear film. This self-perpetuating vicious cycle is the pathophysiological core of dry eye disease.

The TFOS DEWS II (Tear Film & Ocular Surface Society Dry Eye Workshop II) 2017 consensus report is the most cited framework in dry eye research. Its definition emphasises that DED is not a single disease but a spectrum — aqueous-deficient, evaporative, or combined — all converging on the same outcome: a tear film unable to protect and nourish the ocular surface.

Epidemiological data on dry eye in India is remarkably high and remarkably variable — reflecting both the genuine burden of disease and the inconsistency of diagnostic criteria across studies. A 2012 population-based study from Andhra Pradesh reported DED prevalence of 18.4% in adults over 40. A study from Hyderabad reported 32.4%. Studies in contact lens wearers and screen-heavy professions report figures above 50%.

Why India's burden is particularly high:

• Climate: Hot, dry weather in large parts of the subcontinent accelerates tear film evaporation — the same mechanism as meibomian gland dysfunction, compounded by environmental conditions.
• Air pollution: Particulate matter and vehicle exhaust are independently associated with goblet cell loss and tear film instability. Delhi, Mumbai, and dozens of Indian cities regularly exceed WHO PM2.5 limits.
• Screen time: A 2023–24 survey found the average Indian urban adult spent 7.3 hours/day looking at screens. Blink rate falls from 15–20 blinks/minute at rest to 3–5 blinks/minute during screen use — dramatically reducing tear film refreshment.
• Diabetes: India's 77 million diabetics face a specific risk pathway — peripheral neuropathy extends to corneal nerves (diabetic keratoneuropathy), reducing corneal sensitivity and reflex tearing, impairing both tear secretion and surface healing.
• Vitamin A deficiency: While severe deficiency causing xerophthalmia has declined, subclinical deficiency affecting goblet cell density and mucin production remains a factor in lower-income populations.
• Low awareness: Patients present to pharmacists with "eye fatigue" and receive lubricants without diagnosis. Years pass before the condition is formally evaluated.

The DEWS II classification divides DED into two mechanistic subtypes — though in clinical practice, most patients have elements of both. Identifying the predominant mechanism directs treatment, because the interventions for each are meaningfully different.

Because MGD underlies 86% of all dry eye disease, it deserves its own explanation. Meibomian glands are modified sebaceous glands running vertically through the tarsal plates — approximately 25–40 in the upper lid and 20–30 in the lower. Each gland opens through a ductal orifice at the lid margin. Meibum — the complex lipid secretion — is expressed with each blink.

In MGD, two failure modes exist. Hypersecretory MGD: glands produce excessive, abnormally thick meibum that forms plugs at the orifices (meibomian capping). Hyposecretory MGD: glands gradually atrophy (dropout), producing less meibum — confirmed by meibography, which images gland structure through transillumination. Once glands atrophy, they do not regenerate. MGD management is therefore about slowing dropout and maximising function in remaining glands — not reversing damage already done.

Dry eye diagnosis is fundamentally multi-parametric. No single test reliably identifies DED or its severity — the condition is too heterogeneous. The clinical workup integrates symptoms (patient-reported questionnaires), signs (slit-lamp examination, staining, TBUT), functional tests (Schirmer's, osmolarity), and increasingly, imaging (meibography). Diagnosis requires at least one symptom measure and one sign measure to be positive.

TBUT measures tear film stability. A drop of fluorescein is instilled, the patient blinks once, and the examiner times how long before the first dark spot (break) appears in the tear film under blue-filter slit-lamp illumination. Normal: >10 seconds. Borderline: 5–10 seconds. Significant instability: <5 seconds. Non-invasive TBUT (NITBUT), using Placido disc topography or Keratograph without fluorescein, is now preferred as it avoids the confounding effect of fluorescein itself on the tear film.

Fluorescein strips (such as FLUROSCÉNE by Agaaz Ophthalmics) are the standard vehicle for TBUT testing in clinical practice — a single strip is moistened with saline and touched to the inferior conjunctival fornix. The fluorescein disperses into the tear film within seconds, allowing visualisation under cobalt blue filter. The same staining reveals corneal punctate epithelial erosions caused by tear film instability — a direct indicator of surface damage.

A calibrated filter paper strip (Schirmer's strip) is placed over the lower lid margin, folded at the 5mm mark. After 5 minutes (with or without topical anaesthetic for the basic vs reflex secretion variant), the length of wetted paper is measured. Normal: >10mm in 5 minutes. 5–10mm: moderate deficiency. <5mm: severe aqueous deficiency — consistent with Sjögren syndrome when combined with systemic features.

Tear osmolarity above 308 mOsm/L — or an intereye difference above 8 mOsm/L — is considered a positive biomarker for DED. Hyperosmolarity both results from and drives the inflammatory cycle of dry eye. Point-of-care osmolarity testing (TearLab, I-PEN) allows in-office measurement from a microlitre sample taken from the tear meniscus. Osmolarity correlates with disease severity and responds measurably to effective treatment, making it valuable for monitoring.

The DEWS II severity grading classifies DED into four grades based on symptom frequency, visual impact, signs at examination, and corneal/conjunctival staining. The grade determines the appropriate treatment step.

Treatment follows a step-ladder aligned with severity. The principle is to start with the least invasive, most accessible intervention and escalate based on response. Critically, unlike most inflammatory diseases, DED requires long-term management, not episodic treatment. The goal is controlling the inflammatory cycle while addressing the underlying mechanism.

Cataract surgery and dry eye disease intersect in two important directions. First, cataract surgery causes or worsens dry eye in a substantial proportion of patients. Second, pre-existing dry eye worsens outcomes from cataract surgery — including biometry accuracy, patient satisfaction, and post-operative comfort — making its management before surgery a clinical priority.

• Corneal nerve transection: The clear corneal incision (typically 2.2–2.75mm) severs sub-basal corneal nerve plexus fibres in the surrounding tissue. These nerves mediate both corneal sensitivity (triggering reflex tearing when the surface dries) and trophic support to the epithelium. After surgery, corneal sensitivity falls measurably for 3–6 months. During this period, reflex tearing is blunted and epithelial healing is slower — even in eyes that had normal tear function pre-operatively.
• Perioperative preservative toxicity: Topical pre- and post-operative drops (antibiotics, NSAIDs, steroids) contain benzalkonium chloride (BAK) which damages goblet cells and the lipid layer. A standard post-operative regimen of 3 drops 4× daily for 4 weeks delivers significant BAK exposure to an already-compromised ocular surface.
• Surgical surface exposure: Draping the eye, speculum pressure, and the bright operating microscope light all stress the tear film during surgery. Surgical time and irrigation fluid composition matter.
• Pre-existing subclinical DED: Studies consistently show that a significant proportion of cataract surgery candidates have undiagnosed dry eye — subclinical MGD or borderline TBUT that becomes clinically apparent under the stress of surgery.